Plastic dental compositions are increasingly being used for dental restoration in the dental sector. Such plastic dental compositions usually include a matrix of organic resins and various inorganic fillers. Inorganic fillers predominantly comprise powders of glasses, (glass-) ceramics, quartz or other crystalline substances (e.g. YbF3), sol-gel materials or aerosils, which are added to the plastic composition as filling material.
The use of plastic dental compositions is intended to avoid possible harmful side-effects of amalgam and to achieve an improved aesthetic impression. Depending on the plastic dental compositions selected, they can be used for different dental restoration purposes, for example, for tooth fillings, as well as for securing parts, such as crowns, bridges and inlays, onlays etc.
The filling material per se is intended to minimize the shrinkage caused by polymerization of the resin matrix during curing. For example, if there is a strong adhesion between the tooth wall and filling, excessive polymerization shrinkage can lead to the tooth wall breaking. If the adhesion is inadequate, excessive polymerization shrinkage may result in the formation of peripheral gaps between the tooth wall and filling, which can promote secondary caries. Furthermore, certain physical and chemical demands are imposed on the fillers.
It is desirable to process the filling material to form powders that are as fine as possible. The finer the powder, the more homogenous the appearance of the filling. At the same time, the polishing properties of the filling are improved, which in addition to reducing the surface area available for attack also leads to improved resistance to abrasion and therefore to a longer-lasting filling. To enable the powders to be processed successfully, it is also desirable for the powders not to agglomerate. This undesirable effect tends to occur with filling materials produced using sol-gel processes.
Furthermore, it is advantageous if filler particles are coated or at least partially coated with functionalized silane, since this facilitates formulation of dental compositions and improves the mechanical properties.
Furthermore, the refractive index and color of the entire plastic dental composition, including fillers, should be as well matched as possible to the natural tooth material, so that it is as indistinguishable as possible from the surrounding, healthy tooth material. The grain size of the pulverized filler being as small as possible also helps to achieve this aesthetic criterion.
It is also important for the thermal expansion of the plastic dental composition in the typical range of use, i.e. usually between −30° C. and +70° C., to be matched to that of the natural tooth material in order to ensure that dental restoration measures are sufficiently able to withstand temperature changes. Excessively high stresses caused by temperature changes also can cause formation of gaps between plastic dental compositions and the surrounding tooth material, which in turn can form sites of attack for secondary caries. In general, fillers with the lowest possible coefficient of thermal expansion are used to compensate for the high thermal expansion of the resin matrix.
Good chemical resistance of the fillers with respect to acids, alkalis and water and good mechanical stability under load, such as, for example, during movement produced by chewing, can also contribute to a long service life for dental restoration measures.
Furthermore, for the treatment of patients, it is imperative that dental restoration measures can be seen in an X-ray image. Since the resin matrix itself is generally invisible in an X-ray image, the fillers must provide the required X-ray absorption. A filler of this type which provides sufficient absorption of X-radiation is described as X-ray opaque. Constituents of fillers, for example, certain components of a glass, or other substances, are generally responsible for X-ray opacity. Such substances are often referred to as X-ray opacifiers. A standard X-ray opacifier is YbF3, which can be added to the filler in crystalline, milled form.
According to International Standard DIN ISO 4049, the X-ray opacity of dental glasses or materials is quoted in relation to the X-ray absorption of aluminum as aluminum equivalent thickness (ALET). The ALET is the thickness of an aluminum sample which has the same absorption as a 2 mm-thick sample of the material to be tested. An ALET of 200% therefore means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 4 mm. Analogously, an ALET of 150% means that a small glass plate having plane-parallel surfaces and a thickness of 2 mm produces the same X-ray attenuation as a small aluminum plate with a thickness of 3 mm.
Because plastic dental compositions in use are usually introduced into cavities from cartridges and then modeled in the cavities, such compositions should be at least somewhat thixotropic in the uncured state. This means that viscosity decreases when pressure is exerted, while it is dimensionally stable without the action of pressure.
Among plastic dental compositions, a distinction also should be drawn between dental cements and composites. In the case of dental cements, also known as glass ionomer cements, the chemical reaction of fillers with the resin matrix leads to curing of the dental composition, and consequently the curing properties of the dental composition. Thus, their workability is influenced by the reactivity of the fillers. This often involves a setting process which is preceded by a radical surface curing, for example, under the action of UV light. Composites, also referred to as filling composites, contain by contrast fillers which are as chemically inert as possible, since their curing properties are determined by constituents of the resin matrix itself and a chemical reaction of the fillers often disrupts this.
Because glasses, due to their different compositions, represent a class of materials with a wide range of properties, they are often used as fillers for plastic dental compositions. Other applications as dental material, either in pure form or as a component of a material mixture, are also possible, for example, for inlays, onlays, facing material for crowns and bridges, material for artificial teeth or other material for prosthetic, preservative and/or preventive dental treatment. Glasses of this type used as dental material are generally referred to as dental glasses.
In addition to the dental glass properties described above, it is also desirable for this glass to be free from barium and/or barium oxide (BaO), which are classified as harmful to health, and also from lead and/or lead oxide (PbO) and from other barium and lead compounds.
In addition, it is also desirable for a component of dental glasses to be zirconium oxide (ZrO2). ZrO2 is a widely-used material in the technical fields of dentistry and optics. ZrO2 is readily biocompatible and is distinguished by its insensitivity to temperature fluctuations. It is used in a wide variety of dental supplies in the form of crowns, bridges, inlays, attachment work and implants.
Dental glasses therefore represent glasses of particularly high quality. Glasses of this type also can be used in optical applications, particularly if such applications benefit from the X-ray opacity of the glass. Since X-ray opacity means that the glass absorbs electromagnetic radiation in the region of the X-ray spectrum, corresponding glasses simultaneously act as filters for X-radiation. Sensitive electronic components can be damaged by X-radiation. In the case of electronic image sensors, for example, the passage of an X-ray quantum may damage the corresponding region of the sensor or result in an undesirable sensor signal which can be perceived, for example, as an image disturbance and/or disturbing pixels. For specific applications it is therefore necessary, or at least advantageous, to protect electronic components against X-radiation by using corresponding glasses to filter said components out from the spectrum of the incident radiation.
A number of dental glasses and optical glasses are known from the prior art.
For example, WO2005/060921 A1 describes a glass filler which, in particular, is intended to be suitable for dental composites. However, this glass filler must contain only 0.05 to 4 mol % alkali metal oxides. This low alkali metal oxide content of the metal oxides, in particular in combination with ZrO2, makes the dental glass more inclined to segregate. The segregated regions act as centers for scattering light that passes through, analogous to the Tyndall effect. This may create unfavorable consequences for the optical properties of the dental glass and the aesthetics of the plastic dental compositions produced with segregated dental glasses therefore cannot satisfy relatively high demands.
EP 0885606 B1 describes an alkali metal silicate glass which serves as filling material for dental material. The limited B2O3 content of 0.2 to 10% by weight described in this reference makes it difficult to melt glass with a high SiO2 content, making it expensive and uneconomical to produce such glass.
U.S. Pat. Nos. 5,976,999 and 5,827,790 relate to glass-like ceramic compositions used, inter alia, for dental porcelain. These references state that CaO and LiO2 must be present in amounts of at least 0.5% by weight and 0.1% by weight, respectively. In addition to the two main additional components from the group consisting of ZrO2, SnO2 and TiO2, a content of CaO therein of at least 0.5% by weight appears to be essential. These components bring about X-ray opacity and an increased refractive index nd. Even small amounts of CaO enhance the mechanical properties, such as for example the Vickers hardness. However, an increased Vickers hardness is disadvantageous during the milling process since the milling bodies are subjected to increased abrasion and the duration of the process increases.
Certain chemically inert dental glasses for use as filler in composites are described in DE 198 49 388 A1. The glasses proposed therein must contain significant proportions of ZnO and F which can lead to reactions with the resin matrix, which can in turn have effects on their polymerization properties. In addition, the SiO2 content is limited to 20-45% by weight so that such glasses can contain sufficient X-ray opacifier and F.
DE 4443173 A1 describes glass which has a high ZrO2 content of more than 12% by weight and which contains other oxides. Fillers such as these are too reactive, in particular for most modern epoxy-based dental compositions in which excessively rapid, uncontrolled curing may occur. Zirconium oxide in this amount tends to become devitrified. This brings about phase segregation, possibly with nucleation and subsequent crystallization.
WO 2005/080283 A1 describes glass with a refractive index gradient for optical elements. However, the claimed glass contains 12-50% by weight B2O3 which impairs the chemical resistance of the glass and is therefore unsuitable for dental glasses.
US 2003/0161048 A1 describes a glass (lens) with a refractive index gradient. This is achieved by the diffusion of silver. Mandatory for this purpose are at least 3 mol % of the easily exchangeable Li2O and a total Li2O and Na2O content of 3 to 65 mol %. Because it is easy to exchange Li2O, it is desirable in dental glasses and also other weathering-resistant glasses to dispense with the Li2O component. Li2O is quickly leached from the glass and, in the presence of tooth material, may reduce the durability thereof. Furthermore, the glass itself is destabilized by such leaching and the transparency may also be adversely affected. Thus, leaching should be avoided in optical glasses.
DE 3501898 C2 describes glass for optical waveguides, but this glass must contain F. For the reasons already mentioned, F is undesirable in dental glasses.
JP 2006-052125 A2 relates to a silicate substrate glass for flat panel displays which, for adjusting viscosity, contain considerable proportions of alkaline-earth metal oxides, i.e. the sum total of MgO, CaO, SrO and BaO is 15 to 27% by weight. The viscosity curve of this glass is very steep, which means that there is only a narrow temperature window for producing such glass, making production more complicated.
Features common to the glasses mentioned in the prior art are that they either (1) have a relatively high refractive index nd, and/or (2) have low weathering resistance and/or (3) are not X-ray opaque and/or (4) in addition are often difficult or expensive to produce, and/or (5) contain components which are harmful to the environment and/or to health.